Many products or workpieces require multiple machine operations each necessitating a different tool and in many cases a different position of the workpiece with respect to the tool. Much effort has been devoted to solving the problems which arise from this situation. The most fundamental of these problems is tolerance accumulation and misalignment. One of the most common approaches to this problem has been to mount the workpiece on a jig or plate-like support and transport the workpiece so mounted from tool to tool. At each tool means are provided for grasping the support and holding it rigidly in position for the tool to perform its work on the workpiece. If there is any lack of accuracy in this means, the result is a corresponding lack of accuracy in the machining performed on the workpiece. When a workpiece and its support are moved progressively through several stations with one or more machining functions being performed at each station, the deviations from the original design dimensions become cumulative. The result is a high rate of unacceptable product or at least products which, at best, are only marginally acceptable.
One of the methods extensively used for parts requiring multiple machining has been the so-called transfer machine. In this arrangement the part to be machined is mounted on a pallet or plate and, while so mounted, is moved from machine to machine until all of the several machining functions have been performed on it. At each machine clamping devices position the plate and thus the part with respect to the tool. The indexing or locating means normally consists of a pair of locators one of which is a dowel or pin which fits into a precisely-sized opening in either the plate or the plate support and the other is a locator pin which is normally conical to shift or pivot the plate to its final position. The dowel and locator pins are spaced as far apart as possible to improve accuracy. In some cases edge engaging stops and a wedge and V-shaped mating edge opening are substituted for the dowel and locator pin. That part of the indexing means located at each of the tools must be individually fabricated and, therefore there is a certain variation in plate location from station to station due to tolerances in manufacture which cannot be eliminated. Typical of this type of system is that disclosed in U.S. Pat. No. 2,392,169 entitled "Machine Tool" issued Jan. 1, 1946 to J. H. Mansfield. This type of system even if very precisely constructed and carefully maintained cannot eliminate tolerance accumulations which prevent it from attaining repeatable accuracy within the range of 10.sup.-4 inch.
In an effort to overcome the accuracy deficiency of the system previously described, a gear type of indexing or registration means was introduced in which the plate or pallet, on one surface, has a ring of gear-like teeth and the station at which the tool is located is equipped with a matching ring of gear-like teeth designed to mesh with those on the pallet or plate. This arrangement is disclosed in U.S. Pat. No. 3,540,566 entitled "Connector System" issued Nov. 17, 1970 to C. B. Perry et al. In this arrangement the location of the plate with respect to the tool is controlled by the accuracy with which the teeth of the "connector" or indexing locator mesh. This type of indexing or locator arrangement is utilized in the system disclosed in U.S. Pat. No. 3,543,392 entitled "Machine Tools Having Conveyor Means Extending Therebetween and Carrying Pallet Means Which Are Selectively Connectable to the Machine Tools" issued Dec. 1, 1970 to C. B. Perry et al. and again in U.S. Pat. No. 4,309,600 entitled "Machine Tool" issued Jan. 5, 1982 to C. B. Perry et al. This system, at best, can attain an accuracy of 0.0005 of an inch. The thrust of the system is circumferential accuracy. However, it is much less effective in controlling concentricity. To control concentricity, the teeth require very expensive, precision machining which, at best, falls short of the accuracy necessary to produce precise tolerance control. Further, this system requires the workpiece support, that is the pallet or plate, to be used only with tools having the identical registration system, i.e. the tools must all be manufactured by the same company. This latter is necessary because it is virtually impossible to produce registration rings of adequate identity of manufacture unless they are made on the same machines using the same machining techniques.
A further limitation on all of these prior efforts to solve the problem of machining a part at multiple stations has been that the proposed solutions have focused on holding the part or workpiece stationary while the tool is moved as contrasted with moving the workpiece against a stationary tool. This has greatly limited the utility of prior approaches to this problem. For example, none of the presently used systems can be used with a lathe because this requires movement of the workpiece rather than the tool.
Another approach to the problem has been that of machining the part in a "cell" or at a multi-tool work station. This approach is disclosed in U.S. Pat. No. 3,914,853, entitled "Apparatus For Transporting Workpieces" issued Oct. 28, 1975 to K. Jauch. This approach, utilizing a single work station or cell at which multiple machine operations are performed as the part is relocated within the cell to align it successively with each of the tools also does not solve the tolerance accumulation problem. The system disclosed in the Jauch patent moves the part in a circle from plate holder to plate holder to present it in succession to a series of tools arranged in a circle. At each tool the part has to be positioned relative to the tool by one of the various indexing methods previously described with the same tolerance accumulation. While this type of arrangement automates the handling of the parts of workpieces, it does not address the problem of reducing tolerance accumulation.
The difficulty with prior attempts to solve this problem has been the absence of a single, dependable means common to all work stations which can be relied upon at every work station to repeatedly position the workpiece accurately with respect to the tool which is to be applied to it. A major difficulty with prior solution attempts is that, from a cost standpoint, it is not possible to make the means for holding the workpiece support at each work station precisely identical. Even in those cases when it may have been accomplished, the accuracy has been temporary due to the effects of such factors as thermal changes, wear and the presence of extraneous material such as chips on index surfaces.